2. Aberrant Movement: Aberrant movement may be classified as any one of the following: (1) an "instability catch", defined as deviation from the plane of movement during flexion or extension; (2) "thigh climbing", which is defined as using the hands and pushing on the thighs to assist in obtaining an upright trunk position; (3) a "painful arc of motion", when flexing or returning to upright from a flexed spinal position; or (4) "reversal of the lumbopelvic rhythm", where the trunk is first extended and then the hips and pelvis extend to bring the body upright from a flexed position.